Tim:
Beyond the usual (HTTP, mail, DNS servers, etc), what is the average non-admin user going to set up that listens as a server? Admin- users setting up those traditional services ought to know how to manage firewalls, or they ought not to mess around with those services.
Samuel Sieb:
There are a variety of things like file sharing (webdav), media sharing (dlna), remote desktop, various 3rd party or proprietary software, etc.
So, why can't the installation of those applications automatically include an appropriate firewall rule? Better to allow a controlled opening, rather than just open-slather.